ИСТИНА

We have compiled a global 3-D conductivity model of the Earth with a primary goal to be used for realistic simulation of geomagnetically induced currents (GIC). GICs are generated by magnetospheric substorms (occurred in high latitudes) and geomagnetic storms (occurred in mid-to-low latitudes), and pose a potential threat for man-made electric and electronic systems, such as power electric grids and communication lines. Though precise simulation and prediction of GICs within any particular area or power grid requires high-detail regional conductivity grids, the initial problem of recovering of ionospheric source distribution is of global scale and therefore at this step there is a need for a global 3-D conductivity model. Bearing in mind the intrinsic frequency range of the most intense high-latitude geomagnetic disturbances (a few minutes to a few hours), the compiled 3-D model represents the structure in depth range of 0-100 km and has 0.25 x 0.25 degrees lateral resolution. In order to account for longer-period events, the model includes an underlain larger-depth non-homogenous conductivity distribution inferred from GDS data. The top 0-100 km layer covers seawater, sediments, earth crust, and partly lithosphere/asthenosphere. More explicitly the model consists of a series of quasi-spherical layers, whose vertical and lateral boundaries have been specified based on available data, including global maps of bathymetry, sediment thickness, upper and lower crust thicknesses as well as lithosphere thickness. Once the geometry had been specified, each element of the structure was assigned either a certain conductivity value or conductivity versus depth distribution, according to available laboratory data and conversion laws. This a pripori model constructed from non-EM data, was then refined (within some particular regions) by incorporating the surface conductance model of Russia, as well as conductivity models of Fennoscandia, Australia and South-West of the United States. Moreover, a numerical formalism which was developed for compilation of the model allows for its further refining by assimilation of regional 3-D conductivity distributions inferred from the actual EM data.